Chronaxie is defined as the time required for the current that is

• A. Equal to rheobase
• B. Twice rheobase
• C. Half rheobase
• D. Four times rheobase

Answer: (B)

Chronaxie comes from the strength–duration relationship. The threshold current needed to stimulate a tissue depends on how long you apply the pulse: longer pulses require less current, and a very long pulse can trigger with a smaller current called the rheobase. Chronaxie is the specific time that you need for a stimulus whose current is twice that rheobase to still elicit a response. So the correct idea is a pulse duration for a current equal to two times the rheobase.

For context, if the rheobase is 2 mA, the chronaxie would be the time required for a 4 mA current to trigger excitation. The other options don’t fit because they either refer to current amplitude rather than time, or to a different multiple of rheobase that isn’t the standard definition of chronaxie.